An epoxy resin has generally been used as a sealing material of an LED device, and it has been proposed to use a silicone resin as a sealing material instead of the epoxy resin (Patent Documents 1 to 3). The silicone resin is excellent in heat resistance, weather resistance, and discoloration resistance as compared with those of the epoxy resin, so that it has been mainly used particularly for blue LED and white LED.
However, in recent years, the temperature around the LED device has increased with the increase in the amount of electricity supplied to the LED, and even when the silicone resin is used, the sealing material is degraded whereby the problems are generated that cracks are generated or light transmittance is lowered due to discoloration. Under such a background, in recent years, long-term reliability (i.e., heat resistance) in a high-temperature environment is required for a sealing material of the LED device.
As a general silicone material improved in heat resistance, there has heretofore been reported a heat resistant organopolysiloxane composition in which a reaction product of an organopolysiloxane, a carboxylic acid salt of cerium, and a titanium compound or a zirconia compound obtained by subjecting to heat treatment at a temperature of 150° C. or higher is added as an additive to an organopolysiloxane which becomes a base (Patent Document 4), or a silicone gel composition to which the same additive has been added (Patent Document 5). However, the materials disclosed in these Patent Documents are not an addition curable type silicone resin composition which gives a cured product having rubber hardness, and accordingly, these are not a material that can be used for the uses such as a sealing material of the LED device and the like.
On the other hand, in Patent Document 6, a heat resistant silicone rubber composition containing a mixture of rare earth salts of 2-ethylhexanoic acid has been reported, and it has also been reported that a total light transmittance of a sheet with a thickness of 2 mm at a wavelength of 600 nm is 90% or more. However, this heat resistant silicone rubber composition had a problem that light transmittance of short wavelength light around a wavelength of 400 nm was inferior.